Separate tickets means UA is not obligated to do anything if you didn't show up for boarding of the 8:55 flight. But they will probably rebook you on a later LAX-SFO flight since there are plenty of seats between the two airports.
That being said, I don't think you will have any problem making this connection. You can probably get boarding pass for both flights at IAH. Transferring between UA flights at LAX is all within TSA security so it's fairly simple.